Improve SimplePie memory usage

Description

If we are to use SimplePie for importers (see #7400, for example, but also the RSS, Blogger, etc. importers), then the memory usage of the version included in 2.8 will be problematic. We should probably backport changes between r1032 and r1036 of SimplePie. Attached is a patch that does this.

Also, the parse_url() function will go in SP 1.2 eventually, as everything will move over to the IRI class (actually, more likely the parse_url() will remain for backwards compatibility and just call the IRI class).